In the field of clinical chemistry and, in particular, of microbiology, there is an abundance of assays in which large numbers of samples must be treated. It is still the usual practice that the transfer of samples and reagents take place one at a time. Devices have been developed (e.g., the multi-channel Finnpipette.RTM.) by means of which it is possible to transfer several samples or doses of reagents simultaneously. To achieve a sufficiently high speed and accuracy when samples are shifted and diluted is still a great problem. In particular, in pipetting of small liquid quantities on the order of a microliter, the accuracy is limited by the fact that the point of cutting off of the liquid sample, when discharge of suctioned liquid sample is interrupted, cannot be predicted precisely. That is to say the size of the liquid drop remaining at the tip of the pipette is haphazard and unpredictable.
U.S. Pat. No. 2,980,512 discloses a continuously working colorimeter, in which sample is brought through a pipe into a cuvette, and part of the sample flown through the cuvette is fed back into the intake pipe through a feedback pipe, which joins the intake pipe on its wall. The device is not, however, applicable in dilutions and transfers of separate samples, when high precision is needed.